The present invention relates to a method and apparatus for carrying out the continuous countercurrent washing of particulate solids and more particularly to such a method and apparatus which is especially adapted for the continuous countercurrent washing of microspherical nuclear particles.
Continuous countercurrent washing has been commonly employed for washing particulate material such as finely divided solids in order to remove foreign materials. In the context of the present invention, such foreign material may be in any form, for example either a liquid or a solid, which may be dissolved or suspended or otherwise removed from the particulate solids by means of a suitable washing solution.
Within such continuous countercurrent washing processes, it is particularly important to remove the foreign material as completely as possible either for the purpose of recovering the particulate solids or possibly for the purpose of recovering the foreign material itself. In any event, continuous countercurrent washing has been found to be particularly effective for accomplishing the separation. However, various problems have been encountered in the operation of such continuous countercurrent systems. For example, it is particularly desirable to minimize the amount of washing solution employed during the washing process both to conserve the washing solution and to facilitate the removal from spent washing solution of material which is dissolved or otherwise suspended therein during the washing process. In addition, it has been found to be a problem to maintain the rate and duration of contact between the particulate solids and washing solution with the particulate solids first contacting relatively spent washing solution, then passing into contact with successively fresher gradations of the washing solution and finally contacting fresh washing solution just prior to exit of the washed particulate solids from the system.
The method and apparatus of continuous countercurrent washing as contemplated by the present invention is particularly adapted to the washing or cleansing of microspherical nuclear particles in order to remove from the nuclear particles any foreign material tending to remain in combination with the particles after their formation. For example, microspherical nuclear particles or kernels may be produced by a number of techniques.
One method for forming the microspherical nuclear particles is described in U.S. Pat. No. 3,957,933, issued May 18, 1976 to Walter Egli et al. and assigned to the assignee of the present invention. That patent describes a method and apparatus wherein a droplet generator dispenses uniform spherical droplets into a gelling column which contains gelling components to assist in the development of a final spherical configuration for the droplets. The spherical gelled particles from the process may then be dehydrated or otherwise treated under controlled conditions and sintered to develop high density prior to their being coated with a refractory material such as pyrolytic carbon. However, after undergoing this process, the microspherical particles contain ammonium nitrate and remain in the particles after they have been permanently formed into a spherical configuration. Accordingly, it is necessary to remove essentially all of the ammonium nitrate from the nuclear particles before further processing.
In any event, the method and apparatus of the present invention have been found to be particularly effective in the continuous countercurrent washing of particulate solids such as the microspherical nuclear particles described above for a number of reasons discussed in greater detail below.